1. The Field of the Invention
The invention generally relates to lasers. More specifically, the invention relates to Vertical Cavity Surface Emitting Lasers (VCSELs).
2. Description of Related Art
Lasers are commonly used in many modern components. One use that has recently become more common is the use of lasers in data networks. Lasers are used in many fiber optic communication systems to transmit digital data on a network. In one exemplary configuration, a laser may be modulated by digital data to produce an optical signal, including periods of light and dark output that represents a binary data stream. In actual practice, the lasers output a high optical output representing binary highs and a lower power optical output representing binary lows. To obtain quick reaction time, the laser is constantly on, but varies from a high optical output to a lower optical output.
Optical networks have various advantages over other types of networks such as copper wire based networks. For example, many existing copper wire networks operate at near maximum possible data transmission rates and at near maximum possible distances for copper wire technology. On the other hand, many existing optical networks exceed, both in data transmission rate and distance, the maximums that are possible for copper wire networks. That is, optical networks are able to reliably transmit data at higher rates over further distances than is possible with copper wire networks.
One type of laser that is used in optical data transmission is a Vertical Cavity Surface Emitting Laser (VCSEL). As its name implies, a VCSEL has a laser cavity that is sandwiched between and defined by two mirror stacks. A VCSEL is typically constructed on a semiconductor wafer such as Gallium Arsenide (GaAs). The VCSEL includes a bottom mirror constructed on the semiconductor wafer. Typically, the bottom mirror includes a number of alternating high and low index of refraction layers. As light passes from a layer of one index of refraction to another, a portion of the light is reflected. By using a sufficient number of alternating layers, a high percentage of light can be reflected by the mirror.
An active region that includes a number of quantum wells is formed on the bottom mirror. The active region forms a PN junction sandwiched between the bottom mirror and a top mirror, which are of opposite conductivity type (e.g. a p-type mirror and an n-type mirror). Notably, the notion of top and bottom mirrors can be somewhat arbitrary. In some configurations, light could be extracted from the wafer side of the VCSEL, with the “top” mirror totally reflective—and thus opaque. However, for purposes of this invention, the “top” mirror refers to the mirror from which light is to be extracted, regardless of how it is disposed in the physical structure. Carriers in the form of holes and electrons are injected into the quantum wells when the PN junction is forward biased by an electrical current. At a sufficiently high bias current the injected minority carriers form a population inversion in the quantum wells that produces optical gain. Optical gain occurs when photons in the active region stimulate electrons to recombine with holes in the conduction band to the valance band which produces additional photons. When the optical gain exceeds the total loss in the two mirrors, laser oscillation occurs.
The active region may also include an oxide aperture formed using one or more oxide layers formed in the top and/or bottom mirrors near the active region. The oxide aperture serves both to form an optical cavity and to direct the bias current through the central region of the cavity that is formed. Alternatively, other means, such as ion implantation, epitaxial regrowth after patterning, or other lithographic patterning may be used to perform these functions.
A top mirror is formed on the active region. The top mirror is similar to the bottom mirror in that it generally comprises a number of layers that alternate between a high index of refraction and a lower index of refraction. Generally, the top mirror has fewer mirror periods of alternating high index and low index of refraction layers, to enhance light emission from the top of the VCSEL.
Illustratively, the laser functions when a current is passed through the PN junction to inject carriers into the active region. Recombination of the injected carriers from the conduction band to the valence band in the quantum wells results in photons that begin to travel in the laser cavity defined by the mirrors. The mirrors reflect the photons back and forth. When the bias current is sufficient to produce a population inversion between the quantum well states at the wavelength supported by the cavity, optical gain is produced in the quantum wells. When the optical gain is equal to the cavity loss, laser oscillation occurs and the laser is said to be at threshold bias and the VCSEL begins to ‘lase’ as the optically coherent photons are emitted from the top of the VCSEL.
The slope efficiency of a VCSEL can be affected by many variables. Slope efficiency describes the relationship between an incremental increase in emitted power per incremental increase in electrical current passed through the PN junction. For example, low reflection in the bottom mirror, low transmission of the top mirror, and absorption within the top and bottom mirrors can all degrade the slope efficiency of a VCSEL, as can other optical losses such as scattering from the aperture or other features. Throughout this document, descriptions of low and high transmission or reflection apply at a specific wavelength, generally the wavelength at which the VCSEL lases.
Ideally, the slope efficiency of a VCSEL would remain constant during any operating condition. However, for several reasons, VCSEL slope efficiency usually fluctuates as the operating temperature changes, and often decreases as the operating temperature increases. Variations in slope efficiency over temperature hinder a designer's ability to accurately predict the response of a VCSEL, thereby complicating the design process. Some of the fluctuation in slope efficiency can be attributed to details of the active region, but slope efficiency fluctuation also depends on the over-temperature changes in the materials that comprise the top and bottom mirror layers above and below the active region.
In general, for mirrors formed of semiconductor layers, and within the wavelength range where the mirror layers are substantially transparent to the lasing wavelength, the reflectance of the mirror layers increases as the temperature increases. Typically, the reflectance of the top mirror increases at a higher rate than the reflectance of the bottom mirror as temperature increases. Consequently, as temperatures increase, less power is emitted for a given current input, thereby decreasing slope efficiency.
Furthermore, as temperatures increase, the absorption of the mirrors may also increase, which also affects the VCSEL's slope efficiency. Where there are losses due to absorption within the mirrors, it is possible for the slope efficiency to decrease even if the bottom mirror reflectivity increases more than the top mirror reflectivity.
Further compounding the slope efficiency problems that may arise is the manner in which the bottom mirror is constructed. Common practice has been to use bottom mirror periods (i.e., alternating layers of high and low refractive index materials relative to one another) of the number that would provide the highest level of reflectance when operating at room temperature. By maximizing the reflectance at room temperature, a high slope efficiency is achieved at room temperature. By achieving close to 100% reflectance at room temperature, the bottom mirror reflectance is unable to increase significantly as temperature increases. However, in the top mirror, necessarily less reflective in efficient lasers, reflectance may increase as temperatures increase, thereby causing slope efficiency to suffer.
In addition to the reflectivity of a mirror changing with temperature, the wavelength at which maximum reflectivity and transmission occurs also varies with temperature. Some current VCSEL designs have attempted to minimize slope efficiency fluctuation over temperature by intentionally mismatching the wavelengths at which maximum reflectivity is achieved for the top and bottom mirrors. For example, the top mirror may be designed such that maximum reflectivity is achieved at room temperature, and the bottom mirror may be designed to operate at a wavelength slightly higher than the wavelength at which maximum reflectivity is achieved when operating at room temperature. As the ambient temperature increases, the wavelength at which maximum reflectivity is achieved also increases. The VCSEL emission wavelength is also increasing as the temperature increases. Therefore, the reflectivity of the bottom mirror decreases, and the reflectivity of the top mirror may increase. In this manner, the VCSEL is designed in an attempt to minimize maintain or increase slope efficiency as the ambient temperature increases.
The subject matter claimed herein is not limited to embodiments that solve any disadvantages or that operate only in environments such as those described above. Rather, this background is only provided to illustrate one example technology where some embodiments described herein may be practiced.